1. Field of the Invention
The present invention relates generally to epoxy resins that are toughened with thermoplastic materials. Such toughened resins are used to make high performance composite parts. More particularly, the present invention is directed to increasing the resistance of such thermoplastic toughened epoxies to the crazing and cracking that can occur when the cured epoxies are exposed to solvents, such as methyl ethyl ketone (MEK).
2. Description of Related Art
Composite materials are typically composed of a resin matrix and reinforcing fibers as the two primary constituents. Resin matrices that contain one or more epoxy resins as a principal ingredient are widely used. The composite materials are often required to perform in demanding environments, such as in the field of aerospace where the physical limits and characteristics of composite parts are of critical importance.
Pre-impregnated composite material (prepreg) is used widely in the manufacture of composite parts. Prepreg is a combination of uncured resin and fiber reinforcement, which is in a form that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired. It is well known that the relative amount of fibers and resin in a composite part and the distribution of resin within the fiber network have a large effect on the structural properties of the part. Prepreg is a preferred material for use in manufacturing load-bearing or structural parts and particularly aerospace structural parts, such as wings, fuselages, bulkheads and control surfaces. It is important that these parts have sufficient strength, damage tolerance, interlaminar fracture toughness and other requirements that are routinely established for such parts.
The fiber reinforcements that are commonly used in aerospace prepreg are multidirectional woven fabrics or unidirectional tape that contains fibers extending parallel to each other. The fibers are typically in the form of bundles of numerous individual fibers or filaments that are referred to as a “tows”. The fibers or tows can also be chopped and randomly oriented in the resin to form a non-woven mat. These various fiber reinforcement configurations are impregnated with a carefully controlled amount of uncured resin. The resulting prepreg is typically placed between protective layers and rolled up for storage or transport to the manufacturing facility.
Prepreg may also be in the form of short segments of chopped unidirectional tape that are randomly oriented to form a non-woven mat of chopped unidirectional tape. This type of pre-preg is referred to as a “quasi-isotropic chopped” prepreg. Quasi-isotropic chopped prepreg is similar to the more traditional non-woven fiber mat prepreg, except that short lengths of chopped unidirectional tape (chips) are randomly oriented in the mat rather than chopped fibers.
The tensile strength of a cured composite material is largely dictated by the individual properties of the reinforcing fiber and matrix resin, as well as the interaction between these two components. In addition, the fiber-resin volume ratio is an important factor. Cured composites that are under tension tend to fail through a mechanism of accumulated damage arising from multiple tensile breakages of the individual fiber filaments located in the reinforcement tows. Once the stress levels in the resin adjacent to the broken filament ends becomes too great, the whole composite can fail. Therefore, fiber strength, the strength of the resin matrix, and the efficiency of stress dissipation in the vicinity of broken filament ends all contribute to the tensile strength of a cured composite material.
In many applications, it is desirable to maximize the tensile strength property of the cured composite material. However, attempts to maximize tensile strength can often result in negative effects on other desirable properties, such as the compression performance, damage tolerance and resistance to attack by solvents. In addition, attempts to maximize tensile strength can have unpredictable effects on the viscosity, tack and out-life of the resin matrix.
The viscosity of the uncured resin is an important factor that must be taken into consideration when forming prepreg. The viscosity of the resin must be low enough to insure that the resin components can be mixed completely and then impregnated thoroughly into the reinforcing fibers. The viscosity of the resin must also be high enough to insure that the resin does not flow to any substantial degree during storage or lay-up of the prepreg. Resins that do not have viscosities which meet these basic requirements cannot be used to make prepreg. In any attempt to increase strength and/or damage tolerance of a given cured composite material, it is important that the viscosity of the uncured resin remain within acceptable limits.
The stickiness or tackiness of the uncured prepreg is commonly referred to as “tack”. The tack of uncured prepreg is an important consideration during lay-up and molding operations. Prepreg with little or no tack is difficult to form into laminates that can be molded to form composite parts. Conversely, prepreg with too much tack can be difficult to handle and also difficult to place into the mold. It is desirable that the prepreg have the right amount of tack to insure easy handling and good laminate/molding characteristics. In any attempt to increase strength and/or damage tolerance of a given cured composite material, it is important that the tack of the uncured prepreg remain within acceptable limits to insure suitable prepreg handling and molding.
The “out-life” of prepreg is the length of time that the prepreg may be exposed to ambient conditions before undergoing an unacceptable degree of curing. The out-life of prepreg can vary widely depending upon a variety of factors, but is principally controlled by the resin formulation being used. The prepreg out-life must be sufficiently long to allow normal handling, lay-up and molding operations to be accomplished without the prepreg undergoing unacceptable levels of curing. In any attempt to increase strength and/or damage tolerance of a given cured composite material, it is important that the out-life of the uncured prepreg remain as long as possible to allow sufficient time to process, handle and lay up the prepreg prior to curing.
A common method of increasing composite tensile performance is to change the surface of the fiber in order to weaken the strength of the bond between matrix and fiber. This can be achieved by reducing the amount of electro-oxidative surface treatment of the fiber after graphitization. Reducing the matrix fiber bond strength introduces a mechanism for stress dissipation at the exposed filament ends by interfacial de-bonding. This interfacial de-bonding provides an increase in the amount of tensile damage a composite part can withstand before failing in tension.
Alternatively, applying a coating or “size” to the fiber can lower the resin-fiber bond strength. This approach is well known in glass fiber composites, but can also be applied to composites reinforced with carbon fibers. Using these strategies, it is possible to achieve significant increases in tensile strength. However, the improvements are accompanied by a decrease in properties, such as compression after impact (CAI) strength, which requires high bond strength between the resin matrix and fibers.
Another method of increasing composite tensile performance and resistance to damage is to include one or more thermoplastic materials in the epoxy resin matrix. A variety of different thermoplastic materials in a variety of different forms have been used to toughen epoxy resins. For example, see U.S. Pat. No. 7,754,322.
Multiple layers of prepreg are commonly used to form composite parts for structural applications that have a laminated structure. Delamination of such composite parts is an important failure mode. Delamination occurs when two layers debond from each other. Important design limiting factors include both the energy needed to initiate a delamination and the energy needed to propagate it. The initiation and growth of a delamination is often determined by examining Mode I and Mode II fracture toughness. Fracture toughness is usually measured using composite materials that have a unidirectional fiber orientation. The interlaminar fracture toughness of a composite material is quantified using the G1c (Double Cantilever Beam) and G2c (End Notch Flex) tests. In Mode I, the pre-cracked laminate failure is governed by peel forces and in Mode II the crack is propagated by shear forces. The G2c interlaminar fracture toughness is related to CAI. Prepreg materials that exhibit high damage tolerances also tend have high CAI and G2c values.
The cured prepreg must also be resistant to attack by solvents and other chemicals to which the cured composite part may be exposed. A common test to determine solvent-stress interactive effects on cured resins is to strain a cured resin specimen by bending the specimen and then exposing the strained specimen to a given solvent or other chemical for a period of time, which is typically on the order of a few days or more. The specimen is checked for stress cracking and/or crazing at various times during the test period. The specimens are typically strained in bending from 0% to about 2%. The strain varies proportionally to the arc length of the specimen, which is a characteristic of a clothoid curve (spiral). The test apparatus used to induce the clothoid curve to the specimen is known as a “clothoid strain jig”. Use of the clothoid stain jig allows a single test specimen to be bent so as to provide strains over the entire test range.
Resin specimens are considered to be highly resistant to attack by a given solvent if they do not exhibit any cracks when subjected up to a 2% maximum strain in a clothoid strain jig and exposed to the solvent for 7 days at room temperature. In order to be suitable for use in aerospace applications, the cured epoxy resins must be highly resistant to attack by solvents to which the resin may be exposed. It is important that measures taken to strengthen and/or toughen an epoxy resin do not inadvertently reduce the resins resistance to attack by solvents.
Although many existing prepregs are well suited for their intended use in providing composite parts that are strong and damage tolerant, there still is a continuing need to provide prepreg that may be used to make composite parts for structural applications that have high levels of strength (e.g. compression strength), high damage tolerance (CAI) and interlaminar fracture toughness (G1c and G2c) and which exhibit a high resistance to attack by solvents.